1. Field of the Invention
The present invention relates to television systems and more specifically to a method and apparatus for recovering the data encoded in television signals.
2. Related Art
Data is often encoded in television signals. Closed captioning data encoded in the non-display period of television signals is an example of such data. A broadcaster typically encodes the data according to a convention, and the data is usually recovered within television systems. The data is then used depending on the specific purpose for which it is designed. For example, the text corresponding to closed captioning data may be displayed on a display screen of a television system.
Encoding conventions often include a clock run-in signal, which generally includes zeros and ones in alternate positions. For example, before sending closed captioning data, a clock run-in signal containing 16 bits with zeros and ones in alternate positions is sent in the corresponding line. The clock run-in signal is typically used to determine the specific time points at which the television signal is to be sampled, with resulting samples representing the encoded data.
In one prior embodiment, a phase lock loop (PLL) is used to generate a sampling clock synchronized with the clock run-in signal, and the sampling clock is used to sample the accompanying television signal. Unfortunately, accurate synchronization may require expensive components such as crystals, and is thus unsuitable at least in certain environments.
Accordingly, what is required is a cost-effective method and apparatus to accurately recover the data encoded in a television signal.